Thank you dzsergio I did this and it worked flawlessly on 3 different devices, but it's weird that it didn't worked on my s5, I even rooted it and then tryed again, it continues the download from where I "paused the connection" even do I owerwrited with the full zip... I installed sd fix etc, tryed a lot of ways didn't work, I'm looking on the idea of Desmanto, it seems intresting.
I hope you will find a way and tell us how to do this Local Host server Desmanto, that would be great.
So, back to old ways, like dzsergio has explained. Start download, as soon as the progress shows 0% > turn off wifi > replace the zip > turn on wifi > the GT will directly unpacking the zip and installed the langpack immediately. No root needed. Maybe ur s5 is combining both connection (cell + wifi). Try turning off ur data connection before trying it again. And remember the zip file must be from dl.google.com directly. You cannot rezip the existing file/folder and expect it to work. Since the app using SHA-1 to check the zip integrity. Last time, I wasted about a day just to realize this!
The replace zip method leads me into thinking about creating a profile in tasker (since I am learning using it right now), to automate the process. This profile should monitor the notification and the tasks are turn off wifi, replace zip, turn on wifi. It is just a concept, but maybe someone who is skillful at tasker can help us to create the app to automate this.